#include <iostream>
#include <cstring>
#include <algorithm>
#include <cstdio>
#include <cmath>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <set>
#include <bitset>
#include <utility>
using namespace std;

#define mm(a, n) memset(a, n, sizeof a)
#define mk(a, b) make_pair(a, b)

const double eps = 1e-6;
const int INF = 0x3f3f3f3f;

typedef long long LL;
typedef unsigned long long ULL;
typedef pair<int, int> PII;
typedef pair<double, double> PDD;
typedef pair<LL, LL> PLL;
typedef pair<int, LL> PIL;

inline void quickread() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
}

const int N = 110;

string arr[N];

int n;

bool cmp(string s1, string s2) {
    if (s1.size() != s2.size())
        return s1.size() < s2.size();
    return s1 < s2;
}

// 字符串排序
inline void solution() {
    scanf("%d\n", &n);
    for (int i = 0; i < n; i ++ ) 
    {
        getline(cin, arr[i]);
        reverse(arr[i].begin(), arr[i].end());
    }
    vector<char> res;
    sort(arr, arr + n, cmp);

    for (int j = 0; j < arr[0].size(); j ++ ) {
        char ch = arr[0][j];
        int i;
        for (i = 1; i < n; i ++) 
            if (arr[i][j] != ch)
                break;
        if (i >= n) res.push_back(ch);
        else break;
    }
    if (res.size() > 0) {
        reverse(res.begin(), res.end());
        for (auto & c : res) cout << c;
        cout << endl;
    } else {
        cout << "nai" << endl;
    }
}

int main() {
    freopen("input.txt", "r", stdin);
    // quickread();
    solution();
    return 0;
}